218 



THE CIRCULATORY SYSTEM. 



[CH. xvin. 



of bone, and the umbilical vein. The valves of the portal tributarie 

 are very inefficient. 



Lymphatics of Arteries and Veins. Lymphatic spaces are 



Fig. 220. Diagram showing valves of veins. A, part of a vein laid open and spread out, 

 with two pairs of valves. B, longitudinal section of a vein, showing the apposition of 

 the edges of the valves in their closed state, c, portion of a distended vein, exhibiting 

 a swelling in the situation of a pair of valves. 



present in the coats of both arteries and veins. In the external 

 coat of large vessels they form a plexus of more or less tubular 

 vessels. In smaller vessels they appear as spaces lined by endo- 



Fig. 221. A, vein with valves open. B, with valves closed ; stream of blood passing off by 

 lateral channel. (Dalton.) 



thelium. Sometimes, as in the arteries of the omentum, mesentery, 

 and membranes of the brain, in the pulmonary, hepatic, and 

 splenic arteries, the spaces are continuous with vessels which 

 distinctly ensheath them perivascular lymphatics (fig. 222). 



